Electrolytic tin plating



March 7, 1967 R. LOZANO ETAL 3,308,042

ELECTROLYTIC TIN PLATING Filed June 11, 1963 Inventors Robert Loano CogS. flam- Charles C. Marsha" Jack E. Jogce 3a 196M H-kornegs UnitedStates Patent f 3,308,042 7 ELECTROLYTIC TIN PLATlNG Robert Lozano,Hammond, Coy S. Ham, Munster,

Charles C. Marshall, Gary, and Jack E. Joyce, Chesterton, Ind, assignorsto Inland Steel Company, Chicago, Ill., a corporation of DelawareFiled-June 11, 1963, Ser. No. 287,138 1 9 Claims. (Cl. 20428) Thepresent invention relates generally to an electrolytic method ofproducing tin plate,.and-more particularly to a method of producingelectrolytic acid tin plate having superior corrosion resistance,

Most tin plate used in themetal container industry is electrolytic tinplate produced either from an acid plating solution or an alkalinesolution. The acid'electrolytic plating systemis considered by many tobe a faster, simpler and a more economical technique. Therefore, agreater proportion of tin plate is produced by the acid plating systemthan by the alkaline plating system. However, alkaline tin plate whichhas satisfactory special property test values, such as pickle lag, alloytin couple, and iron solution values, consistently demonstrates superiorcorrosion resistance; whereas acid tin plate having similarlysatisfactory special property test values characteristicallydemonstrates only average corrison resistance. Based on thesedifferences, tin plate having superior cor-. rosion resistance isdesignated Grade A plate; whereas tin plate having average corrosionresistance is designated Grade B plate. Grade A electrolytic tin platehas at least one-third better corrosion resistance than hot-dip (CommonCoke) tin plate. Acid electrolytic tin plate is usually rated Grade B,and at present no commercial electrolytic acid tin plate manufacturer inthe United States is consistently able to produce Grade A ac d tinplate.

Prior attempts to produce Grade A electrolytic acid tin plate haveinvolved changes which result in the formation of a more continuous tinlayer by increasing the size of the tin crystal. More recently, aprocedure has been described for producing Grade A acid tin plate inwhich the acid tin plate before flow brightening is subjected to asub-fusion heat treatment to provide a substantially continuous tin-ironalloy layer between the ferrous metal base and the outer free tin layer,and it has been postulated that a continuous tin-iron alloy layer isessential in order to obtain an electrolytic acid tin plate havingsuperior corrosion resistance with most canned products. Theintroduction of a sub-fusion heat treatment step in a continuouselectrolytic tin line prior to flow brightening, however, involves agreat deal of expense and is not feasible in most existing electrolyticacid tin plate lines.

The present invention solves the problem of producing Grade Aelectrolytic tin plate, and particularly acid tin plate, in an extremelysimple and economical manner which has not heretofore been attemptedwith electrolytic tin plate.

It is therefore an object of the present invention to provide animproved and more economical method of consistently producingelectrolytic tin plate having superior corrosion resistance and improvedquality.

It is a further specific object of the present invention to provide animproved method of consistently producing Grade A electrolytic acid tinplate.

Another object of the present invention is to provide a method ofconsistently producing electrolytic acid tin plate having improvedcorrosion resistance which involves 3,308,042 Patented Mar. 7, 1967 aminimum of changes in a continuous production line for converting steelstrips into finished electrolytic acid tin plate.

Still another object of the present invention is to provide a method offorming a steel surface which has a positive, a negative, or nomeasurable pickle lag.

Other objects of the present invention will be apparent to those skilledin the art from the following detailed description and claims when readin conjunction with the accompanying drawing showing a schematic diagramof a preferred embodiment of the present invention,

It has been discovered that Grade A electrolytic acid tin plate can beconsistently produced by providing on the surface of a steel base stripprior to electrolytic tinning a thin diffused layer containing anelement of the group phosphorus, sulfur, or nitrogen, and particularlygood results have been achieved by providing a very thin diffused layercontaining phosphorus.

The incorporation of elemental phosphorus in steel is known to increasethe adherence of a hot-dip metal coating, such as zinc. Also, heretoforeferrous metal surfaces have been treated to incorporate a small quantityof the element phosphorus in the surface layer thereof to effect astrong bond with hot-dip zinc or other hot-dip metal coating. Phosphorushas also been incorporated in metal surfaces for the purpose ofrust-proofing a ferrous metal.

Heretofore, however, no one has discovered that incorporating a verysmall amount of phosphorus in the surface layer of a steel base prior toelectrolytic tinplating, and specifically incorporating phosphorus bydiffusion into the surface of a steel base used for electrolytic acidtin plating, would make it possible without otherwise materiallychanging the electrolytic tin plating procedure to produce consistentlyelectrolytic tin plate, and partic ularly acid tin plate having thesuperior corrosion resistance which is characterized as Grade A tinplate. The foregoing discovery is particularly valuable to theelectrolytic tin plating art because of the insignificant added expenserequired to incorporate the discovery in any standard continuouselectrolytic tin plating line. Thus, the present discovery with verylittle added cost makes it possible to produce consistently commericalGrade A acid tin plate on electrolytic acid tin plate lines whichheretofore have been unable to produce consistently Grade A acid tinplate under the best commercial operating practice known in thiscountry.

i It will be evident from the following detailed description that theinnovation of the present invention comprises incorporating in anelectrolytic tin plating process a simple and inexpensive chemicaltreating step at a stage prior to annealing which provides on thesurface of the steel strip to be electrolytically tin plated a film orcoating containing a reactive phosphate ion or an equivalent phosphoruscompound and thereafter diffusing said coating into the surface of thestrip by heating in a reducing atmosphere. In the preferred embodimentof the present invention the chemical treatment step is carried out byincorporating a small amount of a phosphoric acid or a mixture ofphosphoric acid and a Water soluble phosphate salt, such as tetrasodiumpyrophosphate or sodium acid phosphate, in the final pre-annealingaqueous rinse, prefera-b-ly through the final ore-annealing rinse watersprays, to provide a dilute acidic phosphate treating bath immediatelyfollowing the conventional pre-annealing cleaning treatment and withoutrequiring any other alterations in the metal composition or in themanner of treatment of a Co the said strip before, during or afterelectrolytic acid tin plating.

The phosphorous-containing chemical bath in which the strip is immersedor otherwise contacted preferably comprises a dilute phosphoric acidsolution having a pH below 7 and preferably not substantally lower thanabout pH 2. The preferred range of pH values is from pH 3 to pH 5. Theaqueous phosphorus-containing solution normally contains between about500 and 1500 ppm. phosphate ion and preferably below 1000 p.p.m. On apercentage basis the phosphorus concentration has a range of betweenabout and 0.2% by weight and preferably below 0.1%. The chemicaltreatment with the herein described phosphate ion-containing solutionsprovides on the surface of the strips treated therewith an amount ofphosphorus equivalent to between about .00015 and .00035 gram P0 persquare fot, as shown by analysis of several phosphate treated annealedproducts. The control strip of low carbon steel from the same melt asthe treated test strips and processed in the same way except that thestrip was not exposed to the phosphate-ion containing solution, whenanalyzed in the same manner for phosphorus, had no measurable phosphorusin the surface thereof.

While the chemical treating solution is conveniently provided by meansof addingphosphoric acid (H PO or one of the other phosphoric orphosphorous acids to the continuous anneal pre-furnace final waterrinse, it is also possible to add to the rinse water a water solublephosphate salt, such as sodium acid phosphate (Na HPO sodium dihydrogenphosphate (Naf-I PO or tetrasodium pyrophosphate (Na P O along withsufficient phosphoric acid or other acid of phosphorus to bring the pHof the bath within a range of pH 2 to 6 and preferably to pH 3 to 5. Itis further contemplated that a non-phosphorus acid could be used toadjust the pH of the phosphate containing solution, such as nitric acidor sulfuric acid. When an all aline phosphate salt is used with aphosphorus acid, a higher total phosphate concentration is possiblewithout exceeding the desired pH range. Thus, a total phosphate-ionconcentration of 1500 ppm. can be present in'the treating solution whencomprised of tetrasodium pyrophosphate and sufficient phosphoric acid toadjust the pfl to below 7 and preferably between pH 3 and 5.

If preferred, it is possible to treat the steel strip with the acidphosphate solution or equivalent phosphorus compound at a point prior toannealing other than immediately before the annealing step. For examplethe strip might be immersed or sprayed with an active acidic phosphorussolution or composition in the final water rinse just after the firstacid pickling bath treatment and immediately before the cold rollingmill treatment.

As a further modification of the present invention, it is possible toprovide the required diffused coating or fil-m containing phosphorus onthe surface of the metal strip to be electrolytically tin plated bycontacting the metal strip with a phosphorus-containing gas underconditions which form a substantially continuous and uniform coating orfilm of phosphorus or phosphorus compound on the surface of the stripand diffusing the film into the surface of the strip prior toelectrolytically tin plating. For example, the strip while heated at atemperature of about 1300 F., as when being annealed, can be contactedby phosphine (PH gas which decomposes on the surface of the strip toprovide a film of elemental phosphorus and which diffuses into thesurface of the strip at the annealing temperature. Other atmospheres orgases containing phosphorus which provide a phosphorus-con-tainingsurface film difiusable into the steel strip can also be used in placeof phosphine.

Following the chemical treatment of the steel base strip it is essentialthat the strip having the phosphorus ion-containing film or an ironphosphate film on the surface thereof be heated to a moderately hightemperature in a reducing atmosphere to effect diffusion of the filminto the surface of the strip. The required heating in a reducingatmosphere is most conveniently carried out by passing the strip at thenormal rate of travel (between approximately 800 and 1250 ft. per min.)continuously through a standard continuous annealing furnace having anormal hydrogen-nitrogen reducing atmosphere (5% 1-1 -95 N and at thenormal operating temperature of around 1200-1500 F. Other types ofcontinuous annealing apparatus or equivalent apparatus can be used, suchas an open coil annealing equipment or a continuous normalizer orsimilar equipment employing a heated reducing atmosphere. If desired,however, the coiled strip can be box annealed for a period of from about6 to 48 hours in the presence of a reducing atmosphere, depending on thedegree or depth of diffusion desired or the special properties requiredof the base metal strip.

While it is preferable that the surface coating formed from the acidicphosphorus solution be exposed to a reducing atmosphere at an elevatedtemperature between about 1200 F. and 1500 F. for a period of about 30to seconds, the strip can be heated longer at a lower temperature or fora shorter period at a higher temperature without adversely affecting theresults. It is not necessary to have any particular reducing atmospherein the annealing chamber and any of the commonly available reducinggases can be used, such as hydrogennitrogen, dissociated ammonia, andthe like. Nor is it essential that the atmosphere have a particular dewpoint. The temperature, time of treatment and atmosphere in theannealing chamber is in each instance, however, regulated and controlledso as to effect diffusion of the phosphorus-containing film into thesurface of the strip to form an integral continuousphosphorus-containing diffused film while the said strip remains in theannealing chamber. The phosphorus film preferably is diffused into thestrip to a depth greater than the depth of surfaces, scratches ormarkings which are formed in the strip following annealing and prior totin plating. The depth of the diffused layer is primarily dependent onthe time the strip is held at the elevated diffusing temperature.

In practicing the preferred embodiment of the present invention asschematically illustrated in the drawing, a hot rolled band 5 about 0.1inch thick is continuously immersed in a conventional sulfuric acidpickle bath 10 and passed through a tandem rolling mill 11 to provide afull hard, low carbon, cold rolled steel strip 12 which generally has athickness of between about .005 to .02 inch and is referred to as blackplate. The steel strip 12 traveling at a rate of between about 800 and1250 feet per minute is subjected to a continuous pre-annealing chemicalcleaning treatment at a cleaning station 13 to remove rolling oils andother surface contamination. In the pre-annealing continuous chemicalcleaning treatment, the strip is continuously dipped in an alkalinecleaning bath 14, then passed through an electrolytic alkaline cleaningsolution 15, and finally through a scrubber 16 where the surface of thestrip 12 is contacted by brushes and sprayed to remove any firmlyadhering particles. Each of the foregoing cleaning steps can be any ofthe conventional types presently used in the art.

The cleaned steel strip 12 which is normally given a final pre-annealingwater rinse following the foregoing chemical cleaning treatment beforepassing through a continuous annealing furnace is, in accordance withthe preferred embodiment of the present invention, next chemicallytreated at a treating station 18 by spraying, dipping, wiping, or thelike with an aqueous treating bath 20 comprising a dilute acidic aqueoussolution of a phosphorus compound, preferably containing a phosphate ionor an equivalent phosphorus oxide ion, to provide on the surface of thestrip 12 an integral thin film or coating containing phosphorous, andwith said phosphorus compound being diffusable into the surface of thesteel strip 12 when subjected to elevated temperatures in a reducingatmosphere.

The steel strip 12 preferably has its surface uniformly coated with afilm of an acidic phosphate ion-containing solution by immersing thestrip 12 in the treating bath 20, passing the strip between conventionalsqueegee rolls 21 which remove excess solution, and rapidly drying thestrip by means of conventional gas driers 24 to form the dry film orcoating 22 on the surface of the steel strip 12. The steel strip 12.withthe coating 22 on the surface can be passed directly into the continuousannealing furnace 25, or the treated strip can be placed in a boxannealing chamber (not shown) of any conventional type, if it isnecessary to provide a deeper diffused phosphoruscontaining coating orspecial metal properties are desired. If desired, however, the annealingcan take place at a subsequent time and the strips can be coiled forstorage.

. Thereafter in the preferred embodiment the steel strip 12 having athin dry film or coating 22 of a phosphorus compound on the surfacethereof is continuously passed through a conventional continuousannealing furnace 25 having therein a reducing atmosphere, such as 5%hydrogen and 95% nitrogen, in which the strip 12 remains for the usualperiod of between about 30 and 50 seconds and during which period thephosphorus -com pound is reduced and diffused into the surface of thesteel strip 12 to form a diffused phosphorus-containing surface layer27.

When the steel strip 12 having the phosphorus-containing film diffusedinto the surface thereof is to be electrolytically coated with tin, theannealed strip 12 is temper rolled, as at 28, and then cleaned andpassed through a conventional sulfuric electrolytic pickle bath normallycontaining about 3% by volume sulfuric acid to remove any surface oxideswhich have formed subsequent to annealing or may be passed directly toelectrolytic acid tinning, if no objectionable oxides or contaminationare present on the surface of the strip 12. However, any conventionalacid electrolytic tinning line can be used to tin plate the treatedannealed strip, such as the preferred Ferrostan line 30 which normallyincludes an electrolytic alkaline phosphate dip 31, an electrolyticsulfuric acid pickle 32, cleaning by one 'or more scrubbers 33,immersion in an acid tin plating bath contained in a plurality ofelectrolytic acid tin plating tanks 34, followed by fusion at 35, andchemical treatment, such as a cathodic dichromate treatment at 36. Theresultant acid tin plated strip 12 is thereafter generally oiled in theusual manner and coiled as at 37 or cut into sheets, as desired.

The amount of phosphorus diffused into the surface of the base stripduring the process of the present invention can be accurately determinedby careful chemical analysis, but it is also possible to determine theoverall changes effected by the present process indirectly by means ofsimpler tests which indicate small alterations in the base metal surfacestructure, crystal boundary composition or the like variables which areknown to have an effect on the plating characteristics of a metal strip.One such test which is found to have close correlation with the chemicaltreatment step of the present invention is the pickle lag test. Thepickle lag phenomenon and the methods of determining said values arediscussed by E. L. Koehler in Transactions of the ASM, vol. 44 (1952),p. 1076. It was found by Koehler that some steels will be attacked orpickled at substantially a constant rate throughout the test. However,many steels display an initial period in the early part of the test inwhich the steel is attacked at a considerably lower rate, with the steelthereafter exhibiting a substantially linear or constant final rate ofpickling characteristic of the metal base. The rate of pickling may bedetermined either by weight loss or by measurement of the hydrogenevolved. The initial period of low rate of pickling or increasing 6 rateof weight loss during pickling is called the pickle lag period. Thequantity of metal or surface skin which is removed during the pickle lagperiod, i.e. until the rate of Weight loss becomes substantiallyconstant, is designated as the pickle lag layer. Generally speaking, thedepth of the pickle lag layer may be from about .0001 inch to about .001inch, as calculated from the quantity of metal removed during pickle lagtests.

While the effectiveness of the instant process in pro ducing Grade Aelectrolytic acid tin plate is determined by long term performance packtests therewith, it is possible to predict the corrosion resistance ofthe tin plate toward most materials with reasonable accuracy by means ofcertain laboratory tests. Two such tests which are used to predict theservice life of tin plate are the Modified Iron Solution Value (MISV)and the Alloy Tin Couple (ATC) tests. Both of these tests measure thecorrosion properties of tin plate on the interior of a C011? tainer andthe test results have been shown to have a high degree of correlationwith the actual service life (i.e. pack performance) of the tin platewhen used under highly corrosive conditions.

The Modified Iron Solution Value test measures the amount of irondissolved from tin plate using a specific acid solution under controlledtest conditions with the test specimen having tin previously removed toexpose the iron-tin alloy layer thereof. Grade A tin plate will have aModified Iron Solution Value less than twenty. A complete description ofthe foregoing test is set forth in a Technical Service DivisionMemorandum of the American Can Company, Maywood, Illinois, entitled TheModified Iron Solution Test, ,October 1960. The Modified Iron SolutionValues of Grade A tin plate are less than about 20 and generally about10 or below.

In the Alloy Tin Couple (ATC) test which is less adapted to productioncontrol use but is considered more accurate than the Modified IronSolution Value test, a tin plate specimen stripped of free tin to exposethe alloy layer is coupled to a reference tin electrode and theelectrode immersed in deaereated grapefruit juice to which has beenadded p.p.m. stannous chloride. The current generated between the twoelectrodes is measured after 20 hours and the results are expressed asmicroamperes per square cm. of alloy layer exposed. (See CorrosionResistance of Electrolytic Tin Plate-Part II, The Alloy Tin Test-A NewResearch Tool, G. G. Karnm et al., Corrosion, l7, 84t92t (1961)February.) The ATC values of Grade A tin plate are less than .l0;ta./cm.and generally about .OS a/cm. or below.

The following specific examples are for the purpose of furtherillustrating the present invention but should in no way be construed aslimiting the invention to the particular materials or conditionsspecified.

Example 1 A low carbon steel strip of the type generally designatedblack plate and having a thickness of .01 inch, which is well suited forthe production of electrolytic acid tin plate and adapted for use in themanufacture of tin cans for packing fruit and juices, while moving at arate of about 1000 feet per minute was thoroughly cleaned by aconventional pre-annealing cleaning treatment comprising an alkalinedip, an electrolytic cathodic cleaning step and scrubbing andimmediately thereafter was continuously passed through a phosphoric acidfinal pre-annealing aqueous rinse solution containing sufiicientphosphoric acid (H PO to provide a pH ranging between 2.2 and 3.1. Thephosphoric acid was introduced into the final water rinse through thefinal pre-annealing rinse water sprays 19 which were equipped withproportioning pumps to add continuously the required amount ofphosphoric acid to maintain the pH between about 2 and 3. The steelstrip at a temperature of about F. remained in contact with the dilutephosphoric acid solution also at a temperature of about 180 F. for aperiod of between about 1 and 3 seconds. After passing the strip throughrolls to form a uniform film and drying the strip by a conventionaldryer, the strip was then passed directly into a continuous annealingfurnace maintained at a temperature of about 1350 F. In the annealingfurnace a reducing atmosphere was maintained which comprised about 4%hydrogen and 95% nitrogen. The dew point of the annealing atmosphere wasmaintained between plus 30 F. and plus 45 F. The steel strip having thephosphate ion-containing film on the surface thereof on emerging fromthe annealing furnace had a phosphorus-containing layer diffused intothe surface thereof, exhibited a pickle lag of from to +4 seconds,whereas initially the pickle lag value for the strip Without thephosphate treatment was to +16 seconds. The treated and annealed steelstrip Was then passed at the normal rate of travel through aconventional Ferrostan electrolytic acid tin plating line employing asolution of stannous sulphate in phenolsulfonic acid with conventionaladditives and conventional plating conditions. The resultant fused tinexhibited MISV test values between 3 and 9, ATC test values between .03and .04 ,ua./cm. and was found to be Grade A tin plate.

Example II A low carbon strip of black plate as in Example I travelingat a rate of about 1000 feet per minute after a conventionalpro-annealing cleaning treatment was continuously passed through adilute phosphoric acid final pre-annealing rinse solution having a totalphosphate ion concentration of 500 ppm. (parts per million) and a pH of3.4 with said acid being introduced trdo-ugh the find rinse sprays.Following continuous annealing as in Example l, the steel stripexhibited pickle lags values of 0 seconds. Thereafter the strip was tinplated by a standard electrolytic alkaline tin plating process toprovide a thin tin coating and the resultant tin plate was rated atGrade A tin plate.

Example III A clean low carbon steel strip as in Example I wascontinuously passed through a preannealing final water rinse to whichphosphoric acid had been directly added in an amount sufficient toprovide a total phosphate ion (P0 concentration of 900 ppm. and a pH ofabout 3.5, followed by the standard annealing treatment as in Example Iin which the hydrogen reducing atmosphere had a dew point of plus 45 F.The treated annealed steel strip exhibited a pickle lag of plus 6seconds. The tin plate produced after conventional electrolytic acid tinplating on a Ferrostan line exhibited a pickle lag of about plus 6seconds, Modified Iron Solution Values of between 3 and 9, and ATCvalues of between .03 and .05 ira/cm? Example I V A clean low carbonsteel strip as in Example I was continuously passed through apie-annealed water rinse solution containing sufficient phosphoric acidto provide a total phosphate ion (P0 concentration of 1200 ppm. and a pHof 2.4. Following the standard annealing treatment in Which the reducinghydrogen atmosphere exhibited a dew point of plus 39 F., the treatedannealed steel strip exhibited pickle lag values of between 0 and 3seconds, whereas the strip before the phosphate treatment had pickle lagvalues between 12 and 16 seconds. The Modified Iron Solution Values andthe ATC values of the tin plate produced by conventional electrolyticacid tin plating as in Example I were values indicating Grade A tinplate.

Example V A low carbon steel strip as in Example I was continuouslypassed at a rate of about 1000 feet per second through a preannealingfinal water rinse bath following the conventional pre-annealingcontinuous cleaning with said bath having directly added theretosufllcient phosphoric acid to provide a total phosphate ion (P0concentration of about 1400 ppm. and a pH of about 1.8 and a smallamount of a wetting agent. Following the usual continuous annealingtreatment, Grade A acid tin plate was produced by passing the annealedstrip through a Ferrostan acid electrolytic tin plating line.

Example VI A clean low carbon steel strip in Example I was continuouslypassed through a pro-annealing final water rinse tank solutioncontaining the usual volume of water and to which was added directlywithout using the rinse sprays five liters of phosphoric acid and 25lbs. of tetrasodium pyrophosphate to provide a total phosphate ion (P0concentration of between 900 and 1400 ppm. and provide an initial pH of3.3. After an endless steel strip was continuously passed through thephosphate solution for 2 hours 15 minutes without further addition ofphosphoric acid or phosphate salt, the pH of the solution rose to pH5.6. The phosphate treated strip was then continuously annealed, as inExample I. The portion of the steel strip which Was treated when thesolution had a pH of 3.3 exhibited a piclde lag after annealing of minus3 seconds, and the portion Which was treated when the solution had a pHof 5.6 exhibited a pickle lag of plus 6 seconds. The treated strip afterannealing was subjected to electrolytic alkaline phosphate (2-4oz./gal.) cleaning and electrolytic pickling in a. 5% by volume sulfuricacid aqueous solution, followed by scrubbing to tho-roughly clean thestrip. Thereafter the thoroughly cleaned strip was electrolytically acidtin plated in the conventional manner, as described in Example I. TheModified Iron Solution Values of the acid tin plate product rangedbetween 3 and 9 with with these average test values being lower than anypreviously processed acid tin plate, and the pro-duct exhibited ATCvalues between 0.3 and 0.4 a./cm. The acid tin plate product was ratedas Grade A tin plate.

In the foregoing specific examples particularly good results wereobtained when the final pro-annealing rinse solution had a total ionphosphate concentration below 1000 p.p.m. and with a pH below 6. Incertain of the phosphate chemical treating runs, particularly Where thetotal phosphate ion concentration was about 1000 ppm. and no precautionswere taken to remove any surface solids, a White precipitate which hasbeen identified as a ferric phosphate, probably ferric orthophosphate orferric pyrophosphate, increased in concentration during the chemicaltreatment of the strip and tended to remain on the surface of the stripas a visible deposit. When the steel strip was annealed with asignificant excess amount of ferric phosphate on the surface, as whenthere was no final rinse sprays used to fiush the strip as the stripleft the phosphate treating bath, the electrolytic acid tin plateproduced from the annealed strip having the ferric phosphate on thesurface assumes a dull appearance which is objectionable for some usesof tin plate. The foregoing objectionable result was corrected, however,by subjecting the annealed strip to a more intensive pickling treatmentimmediately prior to electrolytic tinning (see Example VI).

It is postulated that in the foregoing treatment the acid phosphate ionor equivalent phosphorus ion may react with the iron in the surface ofthe strip 12 to form an iron phosphate surface film which may thereafterbe partially or completely reduced by the reducing atmosphere andannealing treatment to form a diffused layer of iron phosphide. It isalso possible, however, that the phosphate ion or other reactivephosphorus compound may be directly reduced to elemental phosphoruswhich might then be diffused into the surface of the steel strip to forman iron-phosphorus alloy during the annealing treatment. Whatever theprecise chemical form of the phosphorus in the surface of the annealedstrip 12, the acid tin plate after conventional surface fusion of thetin to flow brighten has a normal amount of iron-tin alloy formedbetween the surface of the steel strip and the free tin layer. Thus, theiron phosphate which may be reduced to iron phosphide or iron phosphorusalloy diffused in the surface of the strip 12 does not appear to inhibitthe formation of the conventional iron-tin alloy layer, and the improvedresults achieved by the present invention would not appear to be due toany change in amount of the iron-tin alloy layer. The present invention,however, is not-dependent on any theory of operation, since the improvedresults are achieved by employing the herein disclosed combination oftreating steps in the production of electrolytic tin plate, andparticularly acid tin plate.

From the preceding examples and discussion it will also be evident thatthe present invention provides an improved method of controlling thepickle lag of a steel strip to any desired value below the normal picklelag of the strip. In general the decrease in the pickle lag is afunction of the acidity of the phosphate solution (see Example VI). Insome instances where the pH of the phosphate solution is relatively lowor the P concentration relatively high, there is an actual increase inthe rate of pickling above the constant rate of pickle of the basemetal. When the latter phenomenon occurs, the strip is herein designatedhaving a negative or minus pickle lag, as contrasted with a positive orplus pickle lag when the strip has an initial rate of pickle slower thanthe constant rate of pickle of the base metal. It has been observed thata negative or minus pickle lag valuecan be changed to a zero or positivepickle lag value by subjecting the herein treated annealed strip to apickle treatment with sulfuric acid. The herein disclosed chemicaltreatment provides a further means for adjusting or controlling thepickle lag of a metal strip to a desired low or negative value, wherethe art considers this necessary or helpful to provide a more reactiveor improved base for any surface coating to be applied on the strip.

The term acid tin plate, as used herein, designates electrolytic tinplate, either matte or flow-brightened, wherein the tin iselectrolytically deposited onto a steel base from an aqueous acidicsolution. This procedure is old and well known in the art; and ingeneral involves immersing a conventional thin gauge black plate steelstrip in an acidic aqueous solution of stannous ions while making thesteel a cathode and a tin bar immersed in the solution the anode, andpassing a current between the cathode and anode through the solution.The stannous ions being positively charged migrate to the cathodic steelbase and are plated thereon as a matte deposit of metallic tin. The acidbath commonly comprises a solution of stannous sulphate inphenolsulfonic acid with conventional additives, although other acidscan be used, including various combinations of halide salts and halogenacids.

It should be further understood that the herein disclosed improvement inthe process of producing electrolytic tin plate, and particularly acidtin plate, with few exceptions does not obviate the necessity of usingthe art recognized good plating techniques in the several stages of theelectrolytic plating process in order to produce Grade A tin plate.Thus, while it is possible to use in the annealing furnace a reducingatmosphere having a higher dew point than heretofore considered possibleand still achieve good results, it remains important to have the steelstrip thoroughly cleaned and to use proper electrolytic plating bathconditions when employing the present invention for the production ofelectrolytic acid tin plate.

We claim:

1. A process of producing electrolytic tin plate having superiorcorrosion resistance which comprises; contacting the surface of a steelstrip with a fluid containing phosphorus which forms a surface filmcontaining phosphorus on said strip and which at an elevated temperaturein a reducing atmosphere provides a diffused layer containing phosphorusin the surface of said strip, holding said strip with said film at anelevated temperature in a reducing atmosphere until a diffused layercontaining'phosphorus is formed in the surface layer of said strip, andapplying a coating of tin over said diffused layer by immersing saidstrip in an electrolytic tin plating bath; whereby a more economicalelectrolytic tin plate is produced which has corrosion resistanceproperties characterized as Grade A tin plate.

2. A process of producing electrolytic acid tin plate having superiorcorrosion resistance which comprises; contacting the surface of a steelstrip with an acidic solution containing a phosphorus compound which atan elevated temperature in a reducing atmosphere provides a diffusedlayer containing phosphorus, holding said strip at an elevatedtemperature in contact with a reducing atmosphere until a diffused layercontaining phosphorus is formed on the surface of said strip, andapplying a coating of tin over said diffused layer by immersing saidstrip in an electrolytic acid tin plating bath; whereby a moreeconomical electrolytic acid tin plate is produced which has corrosionresistance properties characterized as Grade A tin plate.

3. A process of producing electrolytic acid tin plate having superiorcorrosion resistance which comprises; contacting the surface of a steelstrip with an acidic solution containing a phosphate ion, holding saidstrip having a surface film containing a phosphate ion at an elevatedtemperature in a reducing atmosphere until a diffused layer containingphosphorus is formed on the surface of said strip, and applying acoating of tin over said diffused layer by immersing said strip in anelectrolytic acid tin plating bath; whereby a more economicalelectrolytic acid tin plate is produced which has corrosion resistanceproperties characterized as Grade A tin plate.

4. A process of continuously producing electrolytic acid tin platehaving superior corrosion resistance which comprises; continuouslycontacting the surface of a steel strip with an acidic solutioncontaining a phosphate ion, continuously passing said strip having asurface film containing a phosphate ion through a treating zone heatedto a temperature between about 1200 and 1500 F. and having a reducingatmosphere for a period of between about 30 and 50 seconds until adiffused layer containing phosphorus is formed in the surface of saidstrip, applying a coating of tin over said diffused layer bycontinuously passing said strip through an electrolytic acid tin platingline and fusing the tin coating to flow-brighten the said tin coating;whereby a more economical electrolytic acid tin plate is produced whichhas corrosion resistance properties characterized as Grade A tin plate.

5. An electrolytic acid tin plating process as in claim 4, wherein saidsolution is a solution of a phosphorus acid with a pH below 7 and notsubstantially lower than 2.

6. An electrolytic acid tin plating process as in claim 4, wherein saidsolution is a dilute aqueous solution of phosphoric acid having a pHbetween about 2 and 6.

7. An electrolytic acid tin plating process as in claim 4, wherein thephosphorus content of said diffused layer is equivalent to between about.00015 and .00035 gram P0 per square foot of surface area of said strip.

8. An electrolytic acid tin plating process as in claim 6, wherein saidsolution also contains a water soluble phosphate salt.

9. A process of producing electrolytic acid tin plate having superiorcorrosion resistance which comprises; contacting the surface of a steelstrip with a gaseous fluid containing phosphorus, holding said striphaving a surface filrn containing phosphorus at an elevated temperaturein a reducing atmosphere until a diffused layer containing phosphorus isformed on the surface of said strip, and applying a coating of tin oversaid diffused layer by immersing said strip in an electrolytic acid tinplating bath; whereby a more economical electrolytic acid tin plate isproduced which has corrosion resistance properties characterized asGrade A tin plate.

References Cited by the Examiner UNITED STATES PATENTS Billeter et al.204-29 Keller et al. 148-615 Miller 20429 Ades et al. 20429 Schmidt etal. 1486.15

JOHN H. MACK, Primary Examiner.

T. TUFARIELLO, Assistant Examiner.

1. A PROCESS OF PRODUCTING ELECTROLYTIC TIN PLATE HAVING SUPERIORCORROSION RESISTANCE WHICH COMPRISES; CONTACTING THE SURFACE OF A STEELSTRIP WITH A FLUID CONTAINING PHOSPHORUS WHICH FORMS A SURFACE FILMCONTANING PHOSPHORUS ON SAID STRIP AND WHICH AT AN ELEVATED TEMPERATUREIN A REDUCING ATMOSPHERE PROVIDES A DIFFUSED LAYER CONTAINING PHOSPHORUSIN THE SURFACE OF SAID STRIP, HOLDING SAID STRIP WITH SAID FILM AT ANELEVATED TEMPERATURE IN A REDUCING ATMOSPHERE UNTIL A DIFFUSED LAYERCONTAINER PHOSPHORUS IS FORMED IN THE SURFACE LAYER OF SAID STRIP, ANDAPPLYING A COATING OF TIN OVER SAID DIFFUSED LAYER BY IMMERSING SAIDSTRIP IN AN ELECTROLYTIC TIN PLATING BATH; WHEREBY A MORE ECONOMICALELECTROLYTIC TIN PLATE IS PRODUCED WHICH HAS CORROSION RESISTANCEPROPERTIES CHARACTERIZED AS GRADE A TIN PLATE.